Detection of incident light or photons plays a great role in the analyses and examination of many different materials in numerous different applications. For example, vacuum electronic devices have been used to detect and measure emitted photons. Such vacuum electronic devices generally utilize electron amplification to generate an electron emission sufficient to provide a signal that may be accurately measured. The electron amplification may be accomplished by a means of secondary electron emission.
The secondary electron emission yield has been investigated for a wide variety of materials. A material's secondary electron emission yield quantifies the performance of a material's ability to emit electrons in response to incident electrons and is defined as the ratio of emitted electrons to incident electrons.
For most metals the emission yield is generally limited to maximum values between 1 and 2. That is, between 1 and 2 electrons are emitted by the metal in response to 1 incident electron impinging on the metal surface. The yield can be much higher for oxides, glasses, and semiconductors, but typically electron emission cannot be sustained from these surfaces because of their low electrical conductivity. A further drawback is that metals, oxides, glasses, and semiconductors tend to polarize as a result of secondary emission and eventually repel incident electrons or, in the alternative, suffer electrical breakdown. Thus, it is difficult to measure the secondary emission yield from metals, oxides, glasses, and semiconductors and they are of limited engineering service.
The use of an untreated diamond film deposited on a metal substrate has proven to be a promising secondary electron emitter. Metal surfaces with a diamond film can increase electron amplification by a factor of more than 10. In fact, measurements at the NASA Louis Research Center indicate that the secondary electron yield for a diamond film deposited on the metal substrate can be as high as 45. Additionally, the diamond film is especially useful because it is conductive and able to sustain electron emission without accumulating a charge. The diamond film is also a robust surface that is extremely resistant to abrasion and heat.
One disadvantage of using a diamond film as a secondary electron emitter is that the emission deteriorates under electron bombardment. In order to restore the emitting quality of the diamond film, the diamond film must be exposed to hydrogen gas or it must be annealed in a vacuum. A second disadvantage is that the diamond films that have been subjected to ion sputtering exhibit properties that are similar to carbon, that is, a secondary yield is typically less than 1.
Studies indicate that the above disadvantages can be overcome, but in turn create other disadvantages. Specifically, diamond films exhibit stable continuous emission when operated in a continuous hydrogen atmosphere. However, the presence of gas in some classes of electronic devices is highly undesirable because the gas may become a source of ions that will damage the diamond film surface.